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Emulating 2 Stage Heating

Hi All - if you can bear with this post, questions are at the bottom.

Client has a conventional single stage heating and cooling system with electric furnace and 24VAC interface with a single transformer for heating and A/C. In addition, there is a building providing heating system that can be called with dry contact closure.

The goal is to establish two stage heating with the building provided heat as the primary source.

I am wondering if this can be accomplished using a conventional 24VAC thermostat. The thermostat would be configured for conventional two stage control and wired as follows:

a. Contact closure for the building's system would be provided through a 24VAC relay that we would install. One relay terminal would go to the single stage system's 24VAC transformer and the other to W1, so that the building heat was called for first stage heating.

b. The single stage system heat relay would be connected to W2 so that this system would be called for second stage heat.

c. The rest of the single stage system would wire up normally (i.e. fan to G, cooling to Y1, transformer to RH/RC)

The main issue I see with this is that the thermostat would turn on the fan when only the building heat was in use, and this is not the desired behavior.

To overcome this, I would install a logic board between the thermostat's fan terminal and the fan wire such that current only passed through if W2 or Y1 was closed. In other words, if only the building heat were in use and the thermostat called for the fan, the fan relay would not be energized.

I posted a direct reply, but it was too technical for the open forums so I deleted it. Sounds like you're on the right track though. With enough relays you can accomplish just about anything. I put together a 5-stage heating system once. It was quite a challenge, but in the end it worked well. It consisted of a dual fuel heat pump system (gas/electric) connected to a wood stove, which had a propane burner on the corner and a heat recovery unit on the stack. It used two indoor thermostats mounted side by side.

Tip: If you configure the t-stat for gas heat it won't bring on the fan on a call for heat. Hopefully you can work out the remaining details from there.